1. Field of the Invention
This invention relates to a load control device for controlling a load such as an electric motor or the like applied to a power tool, and more particularly to an improved load control device capable of correcting a conductive angle of an switching element controlled in a phase control fashion in accordance with a variation of a power supply frequency.
2. Discussion of the Related Art
Referring to FIG. 16, there is shown a circuit block diagram of a conventional motor control circuit used for controlling a speed of an AC motor employed in power tools. An AC motor M and a thyrister SCR for controlling a speed of the motor are connected in series across an AC power supply "e". Across the AC power supply "e" there is connected a series circuit of a variable resistance VR and a capacitor C to provide a phase control circuit for a half wave of the thyrister SCR.
Across the capacitor C there are connected a gate and a cathode of the thyrister SCR through a diode D. An on-and-off switch SW is connected across the thyrister SCR to short the same so as to apply a full voltage of the AC power supply to the AC motor M for a full rotational speed.
As a positive half cycle voltage 121 is applied from the AC power supply "e", a voltage charged in the capacitor C varies as shown in a curve 122 according to a time constant with the variable resistance VR. When the charged voltage of the capacitor C reaches the gate-trigger voltage Vg of the thyrister SCR, the thyrister SCR is turned ON and after then a power shown in a hatched domain of the positive half cycle is applied to the motor M for rotations.
As the variable resistance VR is varied to increase or decrease the resistance, the time constant varies so that the inclination of the charging curve 122 of the capacitor C varies downward or upward and a conductive angle of current flowing through the thyrister SCR decreases or increases (a variation to decrease or increase of the hatched domain in the positive half cycle), whereby the rotational speed of the AC motor is controlled.
The conventional motor control circuit described above employing the phase control circuit composed of the variable resistance VR and the capacitor C has the disadvantage that as the line frequency of the AC power supply "e" varies between 50 Hz and 60 Hz, the charging characteristics (the time constant) of the capacitor C does not vary but the conductive angle of the thyrister SCR varies in accordance with the variations of the line frequency whereby a stable motor speed control cannot be expected.
As shown in FIG. 18 at (a), in the respective positive half cycles of a voltage wave form 131 of the line frequency 60 Hz and a voltage wave form 132 of the line frequency 50 Hz, the charging curve 133 of the capacitor C remains constant irrelevant to the line frequencies but the conductive angle of the thyrister SCR varies. When the charged voltage in the capacitor C reaches the gate-trigger Vg of the thyrister SCR, the conductive domain of the thyrister SCR of 50 Hz shown in FIG. 18 at (c) becomes larger than that of 60 Hz shown in FIG. 18 at (b).
Accordingly, as the same quantity of manipulation is applied to the variable resistance VR to change its resistance, the rotational speed of the AC motor at 50 Hz is faster than that at 60 Hz. It is disadvantageous in operations that the same power tool varies with the change of line frequencies. For instance, when the rotational speed of the motor M is controlled by continuously varying the resistance of the variable resistance VR in link motion with the stroke manipulation of a switch employed in the power tool, the relationship between stroke of the switch and rotational speed of the motor in the line frequencies of 60 Hz and 50 Hz is shown in FIG. 19.
There appears a large difference between speed change domains of the line frequencies 60 Hz and 50 Hz, so that the stroke manipulation of the switch must be changed in accordance with the line frequencies, resulting in an inconvenient operation of the power tool.